One prior art implementation of a low-distortion, programmable-gain amplifier is shown in FIG. 1. An input signal, which may be AC or DC, is applied to Vin. The output signal appears at VOUT. In this illustrated arrangement the high-gain operational amplifier A1 is configured as a non-inverting amplifier. This configuration is preferred for low-noise applications over the inverting configuration since the feedback network can be made low impedance to minimize its thermal noise contribution without compromising the amplifier input impedance, which may be set independently via resistor RIN. The feedback network around operational amplifier A1 is tapped at multiple points by electronic switch elements S1 through SN. Each switching element is typically constructed to include complementary metal-oxide semiconductor (CMOS) devices and can be independently turned on or off depending on the desired gain. Control signals (C1 through CN) provided at a control input are used to select the desired gain by turning on the appropriate switch S1 through SN corresponding to the desired gain. Such an approach has the benefit that the variations in on-resistance of electronic switches S1 through SN due to changes in input voltage do not affect the linearity of the output signal since no signal current flows through these switches. This minimizes distortion, so long as one and only one of electronic switches S1 through SN is turned on at any one moment of time.
However, the on-resistance of each of these switches does contribute thermal noise to the total input noise of the amplifier. One way to decrease the on-resistance of CMOS electronic switches (and thus to reduce the amplifier's input noise) is to increase the physical area of the CMOS devices which make up the switches. In an integrated circuit, however, an increase in the area of a switch results in an increased die area. Since the approach illustrated in FIG. 1 requires one switch for each desired gain setting, and since each switch requires at least some die area, the necessary area can be a significant issue.